Mind Hacks

Hack #24. Create Illusionary Depth with Sunglasses

We can use a little-known illusion called the Pulfrich Effect to hack the brain’s computation of motion, depth, and brightness—all it takes is a pair of shades and a pendulum.

This is a journey into the code the visual system uses to work out how far away things are and how fast they are moving. Both of the variables—depth and velocity—can be calculated by comparing measurements of object position over time. Rather than have separate neural modules to figure out each variable, performing the same fundamental processing, the brain combines the two pieces of work and uses some of the same cells in calculating both measures. Because depth and motion are jointly encoded in these cells, it’s possible (under the right circumstances) to convert changes in one into changes in another. An example is the Pulfrich Effect, in which a moving pendulum and some sunglasses create an illusion of the pendulum swinging in ellipses rather than in straight lines. It works because the sunglasses create an erroneous velocity perception, which gets converted into a depth change by the time it reaches your perception. It’s what we’ll be trying out here.

In Action

Make a pendulum out of a piece of string and something heavy to use as a weight, like a bunch of keys. You’ll also need a pair of sunglasses or any shaded material.

Ask a friend to swing the pendulum in front of you in a perpendicular plane, and make sure it’s going exactly in a straight line, left ...